The objectives of this project are to use human adenoviruses (Ad) simian virus 40 (SV40) and Ad-SV40 recombinants to define the role of viral genes and viral antigens in tumor induction by DNA viruses and to study the biology of Ad2-SV40 recombinants. SV40 is highly oncogenic for Syrian hamsters; this virus transforms hamster cells in tissue culture and these transformed cells produced tumors when injected into newborn and adult hamsters. Ad2 is nononcogenic for hamsters; this virus also transforms hamster cells in tissue culture but these transformed cells make tumors only in immunologically immature newborn hamsters or athyumic nude mice. Studies of the tumor-inducing properties of Ad2- and SV40-transformed cells suggest that SV40 conveys to transformed hamster cells the ability to resist rejection by the hamster's cellular immune system. Ad2 appears to be unable to convey resistance to cellular immune rejection to the hamster cell it transforms. These data imply that, in addition to transformation, the induction of some type of resistance to immune rejection is essential to the process by which SV40 converts normal cells to tumor cells. The incorporation of the segment of the SV40 genome which induces this resistance to immune rejection into Ad chromosomes could explain the mechanism by which SV40 conveys increased oncogenicity to Ad-SV40 recombinants.